The present invention relates to an apparatus for detecting organic compounds in ambient air. More particularly, the apparatus is based on a photoionization sensor of sufficiently small size so as to be useful in portable multi-sensor instruments.
The apparatus uses a very small, high-energy, vacuum ultraviolet radiation device, which is attached to a chamber exposed to the ambient air of interest. The chamber is subjected to an electric field. The radiation device is a gas discharge lamp connected to a suitable voltage source. A certain percentage of the organic compounds in this ambient air chamber will become ionized, ie., converted into positively charged ions and negatively charged electrons. The major constituents of the ambient air, such as nitrogen (N2) and oxygen (O2), are unaffected by the radiation device because the energy of the radiation (8.5-11.7 eV) is too low to cause ionization of these constituents. The positive and negative ionization charges are collected by suitable electrodes, thereby generating a current which may be measured to provide an indication of the concentration of organic compounds found in the ambient air.
Therefore, the apparatus is very useful for detection of a wide range of volatile organic compounds in ambient air, in concentrations as low as in the parts-per-billion (ppb) range, without interference from air components.
An ultraviolet radiation source is constructed of a glass housing having a window at one end and being filled with an inert gas such as krypton or argon. A dielectric plate having a pattern of holes drilled proximate its center is placed adjacent the window after a thin metal layer is placed on either side of the plate, and each of the layers is covered with a thin layer of dielectric material. The gas in the lamp housing is excited by a capacitively-coupled radio frequency voltage, causing ultraviolet illumination in the pattern of holes. The apparatus is placed near a source of ambient air containing volatile organic compounds, and the ultraviolet illumination causes ionization of some of the organic compound molecules which have migrated into the pattern of holes. A DC voltage is placed across the metal layers on either side of the dielectric plate, and the charges from the ionized molecules are collected in the metal layers to cause a current to flow; the current is measured to provide a measure of the concentration of volatile organic compounds in the ambient air.